A polyimide film consisting of pyromellitic dianhydride (called PMDA) and diaminodiphenyl ether (called DADE) was manufactured under the name of KAPTON by DuPont in 1960.
On the other hand, a polyimide film called “Upilex” synthesized from biphenyltetracarboxylic dianhydride (called BPDA) and p-phenylenediamine (called PPD) was manufactured by Ube Industries, Ltd. in 1980.
These are widely used as super heat-resistant polymers having good electrical insulating properties, mechanical properties and chemical resistance and characterized by a glass transition temperature of 420° C. or more and a thermal decomposition onset temperature of 500° C. or more.                ((1) Reference: D. Wilson et al; Polyimide, Blackie, New York (1990)).        No alternative super heat-resistant polyimide film to these has ever been marketed since.        
These heat-resistant polyimides are less soluble in solvents so that they cannot be directly synthesized in solution. Thus, a conventional method involves preparing a high-viscosity (therefore, high-molecular weight) compound of an intermediate polyamic acid and then casting and heating it to 400° C. or more to form a film via an imidation reaction while removing the solvent, but the film-forming speed is low. If a third component is added to the polyamic acid solution, an exchange interaction occurs to produce a random copolymer, which is difficult to modify.
No alternative polyimide films to KAPTON and Upilex are commercially available at present. No solvent-soluble block copolymerized polyimide containing components such as PMDA, DADE, BPDA and PPD has been developed.